Interpolation of dynamic three-dimensional maps

ABSTRACT

A method, including acquiring initial signals from selected positions in a heart, computing respective initial local values of a signal propagation metric at the selected positions, and interpolating the initial local values between the selected positions to compute initial interpolated values of the signal propagation metric at intermediate positions, between the selected positions. The method further includes acquiring subsequent signals from the positions, computing respective subsequent local values of the signal propagation metric at the selected positions, and spatially interpolating the subsequent local values of the signal propagation metric between the selected positions to compute subsequent interpolated values of the signal propagation metric at the intermediate positions. A map of the signal propagation metric is displayed, and when the subsequent interpolated values exceed a bound defined with respect to the initial interpolated values, an indication is provided on the map that the bound has been exceeded.

FIELD OF THE INVENTION

The present invention relates generally to representing datagraphically, and specifically to mapping of three-dimensional parametersthat are changing with time.

BACKGROUND OF THE INVENTION

Intra-cardiac ECG (electrocardiograph) signals may be used to track theprogress of a cardiac procedure, such as a ablation procedure.Typically, the signals are acquired by inserting a catheter with one ormore electrodes into the heart undergoing the procedure. The electrodesacquire electropotentials developed in the heart, at the positions wherethe electrodes contact the heart, as the heart beats. The signals maythen be analyzed, and results of the analysis may be used to inform amedical professional performing the procedure of the progress of theprocedure.

The description above is presented as a general overview of related artin this field and should not be construed as an admission that any ofthe information it contains constitutes prior art against the presentpatent application.

SUMMARY OF THE INVENTION

An embodiment of the present invention provides a method, including:

acquiring an initial set of electrical signals from selected positionsin a heart of a subject;

computing, based on the electrical signals in the initial set,respective initial local values of a signal propagation metric at theselected positions;

spatially interpolating the initial local values of the signalpropagation metric between the selected positions to compute initialinterpolated values of the signal propagation metric at intermediatepositions, between the selected positions;

acquiring a subsequent set of electrical signals from the selectedpositions;

computing, based on the electrical signals in the subsequent set,respective subsequent local values of the signal propagation metric atthe selected positions;

spatially interpolating the subsequent local values of the signalpropagation metric between the selected positions to compute subsequentinterpolated values of the signal propagation metric at the intermediatepositions;

generating and displaying a map of the signal propagation metric,including the subsequent interpolated values at the intermediatepositions; and

when the subsequent interpolated values exceed a bound defined withrespect to the initial interpolated values, providing an indication onthe map that the bound has been exceeded.

In a disclosed embodiment the signal propagation metric includes a localactivation time of the heart.

In a further disclosed embodiment there are no intervening sets ofelectrical signals between the subsequent set and the initial set.

In a yet further disclosed embodiment the method includes:

acquiring a prior set of electrical signals from the selected positionsprior to the initial set;

computing, based on the electrical signals in the prior set, respectiveprior local values of the signal propagation metric at the selectedpositions;

spatially interpolating the prior local values of the signal propagationmetric between the selected positions to compute prior interpolatedvalues of the signal propagation metric at the intermediate positions,and wherein the bound is defined with respect to the initialinterpolated values and to the prior interpolated values.

Typically, there are no intervening sets of electrical signals betweenthe prior set and the initial set.

In an alternative embodiment the signal propagation metric includes alocal activation time (LAT) of the selected positions. If there is atime ΔT between the initial set and the subsequent set, the bound may beexceeded when a difference between the LAT at a given selected positionof the initial set and the LAT at the given selected position of thesubsequent set exceeds ΔT.

In a further alternative embodiment the signal propagation metric cyclesin a preset temporal direction, and the bound is exceeded when thesubsequent interpolated values cycle with respect to the initialinterpolated values in a direction counter to the preset temporaldirection.

There is further provided, according to an embodiment of the presentinvention, apparatus, including:

a screen configured to display a map of a signal propagation metric; and

a processing unit, configured to:

acquire an initial set of electrical signals from selected positions ina heart of a subject,

compute, based on the electrical signals in the initial set, respectiveinitial local values of a signal propagation metric at the selectedpositions,

spatially interpolate the initial local values of the signal propagationmetric between the selected positions to compute initial interpolatedvalues of the signal propagation metric at intermediate positions,between the selected positions,

acquire a subsequent set of electrical signals from the selectedpositions,

compute, based on the electrical signals in the subsequent set,respective subsequent local values of the signal propagation metric atthe selected positions,

spatially interpolate the subsequent local values of the signalpropagation metric between the selected positions to compute subsequentinterpolated values of the signal propagation metric at the intermediatepositions;

generate and display on the screen the map of the signal propagationmetric, including the subsequent interpolated values at the intermediatepositions, and

when the subsequent interpolated values exceed a bound defined withrespect to the initial interpolated values, provide an indication on themap that the bound has been exceeded.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, pictorial illustration of a data acquisitionsystem, according to an embodiment of the present invention;

FIG. 2 is a schematic illustration of a map of data presented on adisplay, according to an embodiment of the present invention;

FIG. 3A is a flowchart of steps performed in analyzing acquired data,according to an embodiment of the present invention;

FIG. 3B is a schematic timing diagram of the data, according to anembodiment of the present invention;

FIG. 4 pictorially illustrates a sequence of static data frames,according to embodiments of the present invention; and

FIG. 5 pictorially illustrates a sequence of dynamic data frames,according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

In order to track the behavior of the heart of a subject, sets oftime-varying electrical signals may be acquired from different selectedpositions in the heart, by positioning electrodes at those positions.From the electrical signals a signal propagation metric, such as a localactivation time, may be computed for the selected positions at any giveninstant in time, and the metric for each of the signals in the set maybe used to assess the heart behavior. In order to improve the assessmentvalues of the metric may be calculated for positions intermediate theselected positions, by spatial interpolation between those positions forthe instant of time considered. However, such spatial interpolation maylead to incorrect results, particularly if the heart is not beating in asinus rhythm.

Embodiments of the present invention overcome the problem by comparingspatial interpolations of the metric between different instances oftime. For an initial instance an initial set of signals is acquired atthe selected positions referred to above, the signal propagation metricis computed based on the signals, and initial interpolated values of themetric for positions intermediate the selected positions are calculatedby spatial interpolation between the selected positions.

The procedure is repeated for a subsequent instance, so as to derivesubsequent interpolated values of the metric for the intermediatepositions.

A map of the signal propagation metric is generated and displayed, themap including metric values derived from the signals acquired at theselected positions, as well as the subsequent interpolated values at theintermediate positions. The values are typically indicated by differentcolors within a predefined spectrum of colors.

The subsequent interpolated values may exceed a bound defined withrespect to the initial interpolated values. For example, if the metriccomprises a local activation time (LAT), and the LAT of the initialvalue is −100 ms, the subsequent value cannot be −50 ms unless twoconditions are true: there has been an intermediate value of 0 ms, andthere has been sufficient time between the initial and subsequentinstances for the LAT change to have occurred. When such a bound isexceeded, a visual indication is provided on the map, typically at theintermediate positions where the bound is exceeded. The visualindication may be a color, such as black or white, that is not in thepredefined spectrum of colors referred to above.

The analysis described above is typically used during a playback ofpreviously recorded sets of the electrical signals described above, inorder for a physician to make an overall assessment of the condition ofa heart, typically prior to performing a cardiac procedure such as localablation of a section of the heart. However, the analysis may also beimplemented in a virtually real-time situation, such as during anablation procedure, albeit typically with a delay of one or moreheartbeats to enable the analysis to be performed.

Detailed Description

Reference is now made to FIG. 1, which is a schematic, pictorialillustration of a data acquisition and playback system 20, according toan embodiment of the present invention. As is explained below, system 20may be used to interpolate between sequences of data that have beenacquired at respective sequences of time, and that are subsequently“played back” to an operator of the system. While it will be understoodthat system 20 may be used for substantially any such sequences of data,in the following description, for clarity and simplicity, system 20 isassumed to be applied to electrocardiograph (ECG) data that has beenacquired during a medical procedure on a heart of a subject.

In system 20, a catheter 22 is inserted into a lumen, such as a chamberof a heart 24 of a subject 26 wherein a medical procedure, such asablation of the heart tissue, is to be performed. At a distal end 28 ofthe catheter there are typically a plurality of electrodes, and by wayof example two electrodes 30, 32 are shown in the figure. Typically, thedistal end comprises a larger number of electrodes, but for clarity andsimplicity, such larger number is not shown in the diagram. As isexplained below, at least one of the electrodes at the distal end,herein assumed to be electrode 30 at the tip of the distal end, is usedby system 20. The catheter is manipulated by a medical practitioner 34during the procedure, so as to position electrodes 30, 32 in desiredlocations.

Typically, electrodes at the distal end, including electrodes 30 and 32,may perform multiple functions. For example, an electrode may beconfigured to perform ablation of tissue of the heart, and/or to act asa source electrode for a distal end tracking system operating bymeasuring currents from the electrode. In the description hereinbelow,except where otherwise indicated, electrodes 30 and 32 are assumed toacquire electrical signals, herein also termed ECG potentials, of heart24. Depending on the procedure being performed, the distal end maycomprise other elements. By way of example distal end 28 comprises aposition sensor 36 which, in response to magnetic fields from radiators(not shown in the figure) external to patient 26, generates signalswhich may be analyzed to give the location and orientation of the distalend. U.S. Pat. No. 8,456,182 to Bar-Tal et al., which is incorporatedherein by reference, describes a system using both magnetic fieldmeasurements and currents from a catheter electrode to track thecatheter.

The functioning of system 20 is managed by a system controller (SC) 50,comprising a processing unit 52 communicating with a memory 54, whereinis stored software for operation of system 20. The analysis of thesignals from sensor 36 may be performed by processing unit 52communicating with a catheter tracking module 56. Controller 50 istypically an industry-standard personal computer (PC) comprising ageneral-purpose computer processor. However, in some embodiments, atleast some of the functions of the controller are performed usingcustom-designed hardware and software, such as an application specificintegrated circuit (ASIC) or a field programmable gate array (FPGA).Controller 50 is typically operated by practitioner 34 using a pointingdevice 58 and a screen 60, which enable the practitioner to setparameters of system 20. Screen 60 typically also presents results ofthe procedure to the medical practitioner.

The software in memory 54 may be downloaded to the controller inelectronic form, over a network, for example. Alternatively oradditionally, the software may be provided on non-transitory tangiblemedia, such as optical, magnetic, or electronic storage media.

FIG. 2 is a schematic illustration of a map 100 of data presented onscreen 60, according to an embodiment of the present invention. In thefollowing description, for clarity and by way of example, the datapresented in map 100 is assumed to comprise values of local activationtimes (LATs), of a chamber of heart 24, which are incorporated into athree-dimensional (3D) chart of the chamber. However, in general, thedata presented in map 100 may comprise any signal propagation metricderived from acquired signals, such as a value and/or a time of a localturning point of the signal, or a function of such a value or time.

The measurements for the chart, i.e., the 3D values of points on thewall of the chamber, may have been determined prior to the measurementsof the LATs. Alternatively, the 3D measurements of the wall of thechamber may be determined at substantially the same time as the LATs aremeasured.

The LAT values are typically represented by different colors, that areincorporated into the 3D chart of the chamber, to produce map 100. InFIG. 2 the different colors are shown as different gray scale levels.

The LAT values of any point on the walls of the chamber vary accordingto the beating of heart 24. For any particular instance the LAT valuesare typically measured relative to a fiducial, for example, the time ofa predefined portion of the QRS complex detected by a catheter placed inthe coronary sinus (CS) of the heart.

To acquire the LAT data for map 100, a plurality of electrodes, such aselectrodes 30, 32 and other electrodes not shown in FIG. 1, may beplaced in contact at known positions with the wall of the chamber. ECGsignals from the electrodes are recorded, and analysis of the recordedECG signals provides, for any given instance, the values of the LAT foreach of the known positions. Thus, for any given instance, there is aset of LATs at the known positions. FIG. 2 illustrates examples of knownpositions 102 at which ECG signals are recorded. Typically, there areapproximately 10-100 or even more known positions 102, corresponding tothe number of electrodes recording the signals.

To determine values of LATs for other locations of the walls of thechamber, i.e., at locations of the walls that are not directly measuredby the electrodes and that are intermediate the known positions,embodiments of the present invention use interpolation. Theinterpolation is typically assumed to be implemented during a playbackphase of the operation of system 20, and uses two stages ofinterpolation, which are described with respect to FIGS. 3 and 4.

FIG. 3A is a flowchart of steps performed by controller 50 in analyzingacquired data, FIG. 3B is a schematic timing diagram of the data, andFIG. 4 is a schematic illustration of sets of data generated by thecontroller, according to embodiments of the present invention. The stepsof the flowchart are arranged in a first stage of interpolation followedby a second stage of interpolation. While in the following descriptionthe flowchart steps are assumed to be performed on measurements derivedfrom a sequence of sets of ECG signals that have been recorded at sometime prior to implementation of the flowchart, it will be understoodthat the flowchart may also be implemented in a substantially real-timesituation, albeit with a delay of the order of one heartbeat.

Each set of ECG signals is, as is described above, acquired byelectrodes in heart 24, and each ECG signal in the set is analyzed toprovide a signal propagation metric, herein assumed to be an LAT datavalue, for the position of its acquiring electrode. Thus each set of ECGsignals generates a respective set of measured LAT values for a giveninstance in time of the signals. The flowchart steps are performed on asequence of such sets of LAT values, generated from a correspondingsequence of time instances.

The first stage of interpolation comprises a data recall step 120 and aninterpolation step 122. In step 120 the controller recalls LAT values,for a given instance, that have been stored and acquired at knownelectrode positions. In interpolation step 122 the controller spatiallyinterpolates between the known positions, to determine estimated LATvalues for locations intermediate the known positions. The interpolationis typically linear spatial interpolation, but may comprise any form ofspatial interpolation known in the art. In some embodiments theinterpolation may use known conduction velocities at the positions ofthe electrodes, as well as at positions intermediate the electrodes. Insome embodiments the LAT values for the known positions may be assignedrespective indices indicative of the accuracy and/or the reliability ofthe value, and the indices may be used as weights in implementing theinterpolation. The combination of the known position LAT values and theinterpolated LAT values, for a given instance, is herein termed a staticframe of data.

As indicated by an arrow 124, the controller repeats the procedure ofsteps 120 and 122 for all instances in the sequence being analyzed, togenerate respective static frames of the LAT data.

FIG. 4 pictorially illustrates a sequence of static frames generatedafter step 122 has completed. By way of example, five static frames 150,152, 154, 156, and 158 are shown in FIG. 4, but the sequence of staticframes generated by the first stage of interpolation typically compriseshundreds or thousands of frames. In one embodiment a static frame isgenerated every 1 ms. In an alternative embodiment a static frame isgenerated every 5 ms. The sequence has been positioned relative to atime axis, since successive static frames are for successive instances,herein assumed to occur at successive times T₁, T₂, T₃, T₄, T₅. Whileeach static frame comprises a group of LAT values, the group maytypically be used to generate an image, such as map 100. Thus, in FIG.4, each static frame has been represented as an image presented onscreen 60. However, it will be understood that, rather than presentingstatic frames as illustrated in FIG. 4, embodiments of the presentinvention present other frames, herein termed dynamic frames, on screen60, as is explained below.

Returning to the flowchart, in a comparison step 130, which is a firststep of the second stage of interpolation, the controller selects astatic frame, from those frames produced in step 122, for furtheranalysis. Within the selected frame for analysis (FFA) there aretypically a number of regions having spatially interpolated LAT values.For each of these regions, the controller compares the values of theinterpolated LAT values of the selected static FFA with the LAT valuesof the corresponding region of a static frame subsequent to the selectedFFA. The selected FFA is also herein termed the initial static frame.

In the comparison, for each region where there is interpolated data, thecontroller evaluates if the interpolated values of the subsequent staticFFA are within a bound, which is structured to eliminateinconsistencies, typically timing inconsistencies, between theinterpolated data of the initial static frame and the interpolated dataof the subsequent static frame.

FIG. 3B is a schematic timing diagram of the data for a region, usingLAT values. The diagram illustrates the fact that typically the LATvalues for the region cycle from a value of 0 ms (red) through −50 ms(yellow), −100 ms (green), and −200 ms (blue). At the −200 ms point intime there is a new heartbeat, so that the LAT value begins again at 0ms (red). The cyclic nature of the data is apparent when data isreplayed, with short time periods between sequential frames of data.

The symbol ΔT represents the time period between frames, and this valuecan be selected in the playback process of the frames. Typical values ofΔT for short time periods between frames are 1 ms or 5 ms, and in thesecases the color progression for any particular region is apparent as redthrough yellow, green, then blue, and then red. If the cyclical aspectbetween sequential frames is not maintained, e.g. if a region changesfrom green to yellow, then the bound referred to above is broken.Similarly, even if the cyclical aspect is maintained, but the change inLAT value is too fast or too slow, then the bound is also broken. Forexample, for the case of ΔT being 1 ms, then a change of 10 ms of LATsbetween consecutive frames may be considered too great, breaking thebound. As a second example, for the case of ΔT being 10 ms, then achange of 0 ms of LATs between consecutive frames may be considered toosmall, breaking the bound. The values of the changes of 10 ms and 0 msare exemplary, and may be set by practitioner 34.

While the value of ΔT is typically selected to be small (such as 1 msand 10 ms exemplified above), embodiments of the present inventionencompass larger values of ΔT. A general expression where the boundreferred to above is exceeded is given by expression (1) below.

Assume that for a given region the interpolated LAT value of the initialframe is LAT(T1) and the interpolated LAT value of the subsequent staticframe is LAT(T2). In this case the interpolated values are invalid,i.e., exceed the bound, if the following expression holds:|LAT(T1)−LAT(T2)|<ΔT  (1)

where LAT(T1), LAT(T2)≤0.

Typically, the value for ΔT in expression (1) may be allowed to varywithin an error limit, for example ±10%. Thus, if in a first caseLAT(T1) is −70 ms, LAT(T2) is −30 ms, as illustrated in FIG. 3B, and ΔTis 20 ms, then expression (1) does not hold and the bound is exceeded.However, if in a second case LAT(T1) is −70 ms, LAT(T2) is −60 ms, andΔT is 20 ms, then expression (1) does hold, i.e., is valid.

Another factor that may be used to determine the bound is a refractoryperiod of the tissue i.e., a time period, typically in an approximaterange of 60 ms-100 ms, during which the tissue is incapable ofresponding.

In a decision step 132, the controller proceeds according to theevaluation in step 130. If the evaluation is outside the bound, such asin the first case presented above, the flowchart continues to a firstmap presentation step 134. In step 134 the interpolated data of thesubsequent static frame may be replaced by a visual indication that theinterpolated data is not within acceptable limits. For example theoutside-bound interpolated data may be replaced by a colored region, thecolor being selected so as not to be in the spectrum of colors used forrepresenting the LAT values. For example, the color could be black,grey, or white. In some embodiments, the visual indication may differdepending on the manner in which the bound is exceeded.

If in decision step 132 the evaluation is within the bound, such as inthe second case presented above, the flowchart continues to a second mappresentation step 136. In step 136 the interpolated data of thesubsequent static frame is used as is. Alternatively, the controller mayapply the interpolated data of the initial static frame to adjust valuesof the interpolated data of the subsequent static frame, for example byaveraging, typically in a weighted manner, the two sets of interpolateddata.

Implementation of steps 134 and 136 results in production of a dynamicframe of data, i.e., a frame of data where data of a given static framehas been compared with data of a subsequently acquired static frame.Depending on the result of the comparison, the data of the subsequentstatic frame may be changed to produce the dynamic frame. Typicallycontroller 50 stores the dynamic frame produced in steps 134 or 136 forlater display.

In a final step 138 of the flowchart, the dynamic frame generated instep 134 or step 136 is displayed on screen 60.

The flowchart of FIG. 3 assumes that spatially interpolated values of agiven static frame of data are compared to spatially interpolated valuesof a subsequent static frame of data, and that, depending on the resultsof the comparison, values of the subsequent data frame may be altered.Embodiments of the present invention include other types of comparisonbetween frames of data produced at different times.

For example, rather than a given static frame of data being comparedwith one subsequent static data frame, the comparison in step 130 maycomprise comparison with more than one subsequent static frame of data.Alternatively or additionally, the comparison in step 130 may comprisecomparison of a given static data frame with one or more prior staticdata frames, as well as with one or more subsequent static data frames.Furthermore, in some embodiments there may be one or more interveningframes between the compared static frames where the LAT value is 0 ms.

In all cases, one of ordinary skill in the art will be able to adjustthe definition of the bound used in steps 130 and 132, typically byweighting the values of the prior and/or subsequent data, to reflect thedifferent types of comparison used.

FIG. 5 pictorially illustrates a sequence of dynamic frames generatedafter step 134 or step 136 has completed, according to an embodiment ofthe present invention. Dynamic frames 250, 252, 254, 256, and 258respectively correspond to static frames 150, 152, 154, 156, and 158 ofFIG. 4, are produced after each of the static frames has been analyzedin the flowchart of FIG. 3, and are displayed on screen 60 onimplementation of step 138 of the flowchart.

By way of example, frames 250, 252, and 256 are assumed to have beengenerated in step 136, so that the frames are respectively generallysimilar to frames 150, 152, and 156. Frame 254 is assumed to have beengenerated in step 134, wherein after the analysis of the step a region260 has been found to be outside the bound defined in step 130, and avisual indication 262 has been incorporated into the region. In additionframe 258 is also assumed to have been generated in step 134. In thiscase a region 264 and a region 266 have been found to be outside thedefined bound, and visual indications 268 and 270 have respectively beenincorporated into these regions.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and subcombinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

The invention claimed is:
 1. An electrophysiology method, comprising:acquiring an initial set of electrical signals from tissue at selectedpositions in a heart of a subject, the electrical signals beingrepresentative of tissue conductivity; determining, based on theelectrical signals in the initial set, respective initial local valuesof a signal propagation metric at the selected positions for an initialtime; spatially interpolating the initial local values of the signalpropagation metric between the selected positions to determine initialinterpolated values of the signal propagation metric at intermediatepositions, between the selected positions, for the initial time;acquiring a subsequent set of electrical signals from the tissue at theselected positions; determining, based on the electrical signals in thesubsequent set, respective subsequent local values of the signalpropagation metric at the selected positions for a subsequent time;spatially interpolating the subsequent local values of the signalpropagation metric between the selected positions to determinesubsequent interpolated values of the signal propagation metric at theintermediate positions, for the subsequent time; generating an initialstatic frame based on the initial local values and the initialinterpolated values; generating a subsequent static frame based on thesubsequent local values and the subsequent interpolated values;generating dynamic frames based on the initial and subsequent staticframes in accordance with a rate of static frame generation, comprisingproviding a visual indication in the dynamic frames when a subsequentinterpolated value exceeds a bound defined by the rate of static framegeneration, wherein the bound is exceeded when a difference between aninitial interpolated value of an intermediate position of the initialtime and a subsequent interpolated value at the same intermediateposition of the subsequent time exceeds the rate of static framegeneration; and energizing an ablation electrode to ablate tissue at alocation of the heart to alter the tissue conductivity at the locationbased on the dynamic frames.
 2. The method of claim 1, wherein thesignal propagation metric comprises a local activation time (LAT) of theselected positions.
 3. The method according to claim 1, wherein thereare no intervening sets of electrical signals between the subsequent setand the initial set.
 4. The method according to claim 1, furthercomprising: acquiring a prior set of electrical signals from theselected positions; determining, based on the electrical signals in theprior set, respective prior local values of the signal propagationmetric at the selected positions for a prior time; spatiallyinterpolating the prior local values of the signal propagation metricbetween the selected positions to determine prior interpolated values ofthe signal propagation metric at the intermediate positions for theprior time; generating a prior static frame based on the prior localvalues and the prior interpolated values; and generating dynamic framesbased on the initial and prior static frames, including providing avisual indication in the dynamic frames when an interpolated valueexceeds a bound defined by the rate of static frame generation, whereinthe bound is defined with respect to the initial interpolated values andthe prior interpolated values, wherein the bound is exceeded when adifference between an initial interpolated value of an intermediateposition of the initial time and a prior interpolated value at the sameintermediate position of the prior time exceeds the rate of static framegeneration.
 5. The method according to claim 4, wherein there is nointervening set of electrical signals between the prior set and theinitial set.
 6. An electrophysiology apparatus, comprising: a displayscreen; and a controller having a processor, the controller configuredto: acquire an initial set of electrical signals from tissue at selectedpositions in a heart of a subject, the electrical signals beingrepresentative of tissue conductivity; determine, based on theelectrical signals in the initial set, respective initial local valuesof a signal propagation metric at the selected positions for an initialtime; spatially interpolate the initial local values of the signalpropagation metric between the selected positions to determine initialinterpolated values of the signal propagation metric at intermediatepositions, between the selected positions, for the initial time; acquirea subsequent set of electrical signals from the tissue at the selectedpositions; determine, based on the electrical signals in the subsequentset, respective subsequent local values of the signal propagation metricat the selected positions for a subsequent time; spatially interpolatethe subsequent local values of the signal propagation metric between theselected positions to determine subsequent interpolated values of thesignal propagation metric at the intermediate positions, for thesubsequent time; generate an initial static frame based on the initiallocal values and the initial interpolated values; generate a subsequentstatic frame based on the subsequent local values and the subsequentinterpolated values; generate dynamic frames based on the initial andsubsequent static frames in accordance with a rate of static framegeneration, including providing a visual indication in the dynamicframes when a subsequent interpolated value exceeds a bound defined bythe rate of static frame generation, wherein the bound is exceeded whena difference between an initial interpolated value of an intermediateposition of the initial time and a subsequent interpolated value at thesame intermediate position of the subsequent time exceeds the rate ofstatic frame generation; and energize an ablation electrode to ablatetissue at a location of the heart to alter the tissue conductivity atthe location based on the dynamic frames.
 7. The apparatus of claim 6,wherein the signal propagation metric comprises a local activation time(LAT) of the selected positions.
 8. The apparatus according to claim 6,wherein there are no intervening sets of electrical signals between thesubsequent set and the initial set.
 9. The apparatus according to claim6, wherein the processor is configured to: acquire a prior set ofelectrical signals from the selected positions; determine, based on theelectrical signals in the prior set, respective prior local values ofthe signal propagation metric at the selected positions for a priortime; spatially interpolate the prior local values of the signalpropagation metric between the selected positions to determine priorinterpolated values of the signal propagation metric at the intermediatepositions for the prior time; generate a prior static frame based on theprior local values and the prior interpolated values; and generatedynamic frames based on the initial and prior static frames, includingproviding a visual indication in the dynamic frames when an interpolatedvalue exceeds a bound defined by the rate of static frame generation,wherein the bound is defined with respect to the initial interpolatedvalues and the prior interpolated values, wherein the bound is exceededwhen a difference between an initial interpolated value of anintermediate position of the initial time and a prior interpolated valueat the same intermediate position of the prior time exceeds the rate ofstatic frame generation.
 10. The method according to claim 9, whereinthere is no intervening set of electrical signals between the prior setand the initial set.